Interpretive Summary: Pollution of fresh waters by agricultural phosphorus is a water quality concern. Because soils contribute to phosphorus loss in runoff, it is important to assess how fertilizer, manure and crop management affect soil phosphorus over time. We developed and tested the ability of the Annual Phosphorus Loss Estimator (APLE) computer model to simulate soil phosphorus dynamics over time with data from 25 published studies. Results show APLE reliably simulates soil phosphorus dynamics for a wide range of soil properties, soil depths, phosphorus application sources and rates, number of years, soil phosphorus contents, and management practices. This demonstrates APLE’s potential to be applied to major management scenarios related to soil phosphorus loss in runoff and erosion, which will be useful to nutrient management planners who help producers develop best management practices for manure, fertilizer and crop management.

Technical Abstract:
Pollution of fresh waters by agricultural phosphorus (P) is a water quality concern. Because soils can contribute significantly to P loss in runoff, it is important to assess how management affects soil P status over time, which is often done with models. Our objective was to describe and validate soil P dynamics in the Annual P Loss Estimator (APLE) model. APLE is a user-friendly, spreadsheet model that simulates P loss in runoff and soil P dynamics over ten years for a given set of runoff, erosion, and management conditions. For soil P dynamics, APLE simulates two layers in the topsoil, each with three inorganic P and one organic P pool. It simulates P additions to soil in manure and fertilizer, distribution among pools, mixing between layers due to tillage and bioturbation, leaching between and out of layers, crop P removal, and loss by runoff and surface erosion. We used soil P data from 25 published studies to validate APLE’s soil P processes. Results show APLE reliably simulated soil P dynamics for a wide range of soil properties, soil depths, P application sources and rates, durations, soil P contents, and management practices. We validated APLE specifically for situations where soil P was increasing from excessive P inputs, where soil P was decreasing due to greater outputs than inputs, and soil P stratification in no-till and pasture soils. Successful simulations demonstrate APLE’s potential to be applied to major management scenarios related to soil P loss in runoff and erosion.